Electronic ovens



P. H. SMITH Jan. 24, 1967 ELECTRONIC OVENS 2 Sheets-Sh-zei Filed July 9, 1963 INVENTOR.

7 9% 5/ ?BY MM 4 1967' P. H. SMITH 3,300,615

ELECTRONIC OVENS Filed July 9, 1963 2 Sheets-sheet 2 o o g cf 6;? 62 o o o United States Patent 3,300,615 ELECTRONIC OVENS Peter H. Smith, Maidenhead, England, assignor to J.

Lyons & Company, Limited, London, England, a corporation of England Filed July 9, 1963, Ser. No. 293,688 7 Claims. (Cl. 219-1055) This invention relates to electronic ovens and more particularly to relatively small electronic ovens powered by microwave radiation, and which are suitable for cooking a variety of different foods very rapidly.

In the prior art, electronic ovens have been known which have been powered by microwave radiation, but these have generally been fairly large, and have been designed to permit the maximum number of modes to exist within the oven compartment. In previous attempts to design a relatively small electronic oven, suitable for cooking hamburgers or the like, it has not been possible to attain sufiiciently good impedance matching with the source of RF energy to apply a high proportion of power to the load. It has been supposed by those skilled in the art that relatively small cooking chambers are excessively reactive, and therefore impossible to match to the source. Accordingly, it is the principal object of the present invention to provide a relatively small electronic oven having means for causing substantially equal intensity of field to exist within an oven compartment when only relatively small number of modes of radiation exist within the compartment, and for matching the impedance of the load to the impedance of the source.

Another object of the present invention is to provide an electronic oven having means for causing cooking odors and other vapors created during the cooking process to be expelled by a stream of air flowing through the oven compartment which enters the oven compartment at the same location as themicrowave radiation.

A further object of the present invention is to provide an electronic oven having a novel means for rotating material to be cooked to make the average field intensity even more uniform, and to provide interlock means for detecting when no material to be cooked is present in the cooking chamber.

These and other objects and advantages of the present invention will be manifest upon an examination of this specification and the accompanying claims and drawings.

In one embodiment of the present invention there is provided an electronic oven comprising a source of electromagnetic radiation including an air cooled magnetron oscillator having a plurality of cooling fins. Fan means is disposed in relation to the cooling fins so as to direct a stream of air over the cooling fins toward a wave guide which connects the output of the magnetron with the oven compartment. A'plurality of apertures in the wave guide are disposed at a location corresponding to a point where the air outside the wave guide is at an elevated pressure, and the air thereby flows through the apertures and through the wave guide and into the oven compartment. The oven compartment is provided with a wall having a plurality of apertures to permitfthe stream of air to escape from the oven compartment, carrying with its vapors created in cooking and the like. A slot is provided in the wave guide in alignment with a corresponding slot in one of the walls of the oven compartment whereby energy is transmitted from the wave guide into the oven compartment. A baffle is provided within the oven compartment spaced from the wall in which the slot is disposed, and the baflie serves to spread the electromagnetic energy 3,300,615 Patented Jan. 24, 1967.

flowing through the slot outwardly along the surface of the battle to the outward edges of the baffle plate. The radiant energy is thereby introduced into the cooking chamber substantially uniformly over the periphery of the baflie late, thus operating to create a field intensity within the cooking chamber which is substantially uniform. The baflie also operates to match the impedance of the load to that of the magnetron. In order to aver-age the effect of slight field non-uniformities, a turntable is provided within the cooking chamber upon which the food is supported during the cooking process, and the food is thereby rotated while being cooked to cause substantially uniform cooking of the entire mass of food. In addition, the oven compartment is provided with interlock means which prevents operation of the magnetron when no food is in position within the cooking chamber, and when the door of the oven compartment is open.

Reference will now be made to the accompanying drawings in which:

FIG. 1 is a perspective view of apparatus constructed in accordance with the present'invention;

FIG. 2 is a vertical cross section taken through a portion of the apparatus illustrated in FIG. 1;

FIG. 3 is a rear view of the oven compartment of the apparatus illustrated in FIGS. 1 and 2;

FIG. 4 is a plan view of the apparatus illustrated in FIGS. 1 and 2; and

FIG. 5 is a vertical section of a portion of the apparatus of FIG. 1.

Referring now to the drawings, there is illustrated an oven compartment 10 comprising a rectangular shaped box composed of sheet metal or the like, and having one open end defined by a peripherally extending flange 12. The oven is preferably enclosed in a cabinet or the like 14, part of which is illustrated in FIG. 2, and a hinge 16 is secured to a portion of the cabinet 14 and to a door 18 to permit the door 18 to be selectively opened and closed to facilitate the entry and withdrawal of food into the oven compartment 10. A latch mechanism having a handle 20 maintains the door in its upper, or closed, position.

The food within the oven compartment 10 is cooked by dielectric heating induced by high frequency microwaves which are introduced into the oven 10 through a wave guide 22 having a slot 24 cooperating with an aligned slot 26 in the top wall 27 of the oven compartment 10. The slot 24 is disposed in the wave guide 22 at a location corresponding to maximum field intensity within the wave guide.

Disposed directly below the slots 24 and 26 is a sheet metal baffle plate 28 suspended from the upper wall of the oven compartment by a pair of dielectric spacers 30, and having its edges spaced from the interior of the side and end walls by about A of a wave-length of the frequency being applied.

The function of the baflie plate 28 is to spread the radiant energy passing through the aligned slots 24 and 26 substantially evenly around the periphery of the interior of the oven, thereby to equalize the field intensity within the oven. Food to be cooked is placed on a platform 32 within the oven compartment 10, which is rotatably mounted by bearings 34 on a supporting plate 36, which in turn is supported by a pair of rods 38 extending through the rear wall 39 of the oven compartment 10. The turntable 32 is secured to a shaft 40 extending through the bottom wall 41 of the oven compartment 10, and having a pulley 42 secured to its lower end whereby the shaft 40 and the turntable 32 may be rotated by motive means including a belt or the like connect-ed with the pulley 42.

The presence of the baffle plate 28 also permits the load within the relatively small oven compartment to be closely matched to the impedance of the source.

In operation of the oven, food is placed on the turntable 34 and rotated while it is exposed to microwave energy which enters the oven through aligned slots 24 and 26. Despite the evening effect of the baflie 28, variations in the field intensity within the oven compartment exist which might account for nonuniform cooking of the food within the oven compartment. When the turntable is r-otated, however, the field intensity is averaged over the surface of the food being cooked. Moreover, the rotation of the turntable 32 with the food in place on top of it, presents a geometry which varies with the rotation of the shaft 40, unless the food is placed on the turntable 32 in a way in which it is axially symmetric with the shaft 40. The variations in geometry, resulting from the turntable movement, cause slight changes in load impedance which, in turn, result in slight changes in the frequency generated by the magnetron, and as a result, the orientation of the field is shifted within the cooking chamber. In addition, the dielectric properties of the food generally change during the cooking process. This is particularly true when the moisture content of the food changes considerably during the cooking process. This also affects the load impedance as seen by the magnetron, operating to make changes in the frequency generated by the magnetron thereby to shift the field orientation. The result of all of these factors is to permit food to be cooked within the oven, substantially uniformly throughout the volume of the food.

It has been found that the size of the baffle 28 is somewhat critical, small differences in size giving rise to corresponding differences in performance, but the optimum size baffle and its optimum position can be found easily by trial and error, the optimum distance between the edges of the baffle plate 28 and the side walls 43 and the end walls 39 and 84 of the oven compartment being on the order of A wave length of the frequency of the I input microwave power.

The supports '30 for the baffle plate 28 are preferably formed of dielectric material to prevent baffle plate 28 and the supports 30 from forming a closed loop with the top wall 27 of the oven compartment, which would be magnetically coupled with the field of the oven compartment to cause extreme heating of the metal parts. The use of dielectric material for the supports 38 substantially prevents this effect.

A magnetron 44 furnishes the microwave energy required to cook food within the oven compartment, and is preferably dispose-d behind the oven compartment 10, and is connected with the wave guide 22 with the central conductor 45 of its coaxial output connector connected to a radiating stub 48 inserted within the wave guide 22, to couple the energy generated by the magnetron 44 to the wave guide 22. The outside conductor 46 is electrically connected to the wave guide 22. The wave guide 22 is preferably domed at the side wall directly opposite the location at which the magnetron 44 is connected, to provide a broad band energy coupling, which is substantially insensitive to variations in the load impedance which :may be encountered for different kinds of food and for different impedances during the cooking process. The magnetron 44 is preferably of the air-cooled type having a plurality of fins 48, and a stream of cooling air is forced past the radiating fins 48 by a 'fan having a motor 50 and fan blades 52. The wave guide dimensions are such that only the dominant mode TE can exist within the wave guide.

' The wave guide 22 is provided near its end with a plurality of apertures 54 (FIG. which are in the path of stream of air caused to flow by the fan blades 52. The air moved by the fan blades therefore flows into the wave guide 22 through the apertures 54, and moves along the wage guide 22 and into the oven compartment 10 through the aligned slots 24 and 26. The air stream is permitted to escape from the oven compartment 10 through the rear wall 39 of the oven compartment, which is provided with a rectangular opening 58, closed by an overlapping screen 60 bolted to the rear wall 46 by bolts 62. The screen 60 is provided with a multiplicity of holes 64, each of which is sufficiently small as to prevent radiation from escaping within the oven compartment 10- through the screen 60. The air which is forced through the wave guide 22, however, moves freely through the holes 64 in the screen 60 and carries with it cooking odors and other unwanted vapors generated in the cooking process.

An angle 66 is bolted to the rear wall 39 of the oven compartment 10 by bolts 67, and supports a pair of pivot members 68 which are in the form of bolts held in fixed position with respect to the angle 66 by nuts 70 threaded thereon on the opposite side of the angle 66 from their bolt heads. The upper ends of the pivot members 68 are sharpened to pointed ends which cooperate with collars 72 which are secured to the supporting rods 38. A counterweight in the form of a bar 74 is secured to the ends of each of the supporting rods 38.

The platform 36 is pivotally mounted within the oven compartment 10, and is adapted to pivot about the pivot members 68, in response to the weight of the food which may be placed within the oven compartment 10. When no food is within the oven compartment 10, the position of the support plate 36 is in its upper position, as illustrated in FIG. 2, but when food is placed within the oven compartment, the weight of the food causes the pivot assembly to rotate in a counterclockwise direction as viewed in FIG. 2, about the pivot members 68. The driven connection with the pulley 42 is sufiiciently flexible to account for slight movements in the pulley in response to the movement of the support plate 36.

A microswitch 76 is secured to the rear wall 56 and the oven compartment 10, and its operating element is engaged with the counterweight 74 in the position shown in FIGS. 2 and 3, when no food is present within the compartment 10. When the assembly pivots in response to food being placed in the oven compartment, the counterweight 74 rises out of contact with the actuator of the microswitch 76, thereby to furnish an electrical signal to indicate that the food is in position within the oven compartment 10. The microswitch 76 is preferably connected in circuit with the magnetron 44, to disconnect its anode potential or otherwise disable the magnetron when no food is in position within the oven compartment 10. This is necessary to prevent an inordinately high standing wave ratio which might arise without any dielectric load in the oven, and which tends to minimize the life of the magnetron or otherwise impair its operation.

When the cooking operation is in progress, it is important that the door 18 prevent any substantial amounts of radiation from escaping from the oven compartment 10. This is because such radiation is not only dangerous to humans in large doses, but also its fourth harmonic interferes with aircraft navigation aids. The door 18 best illustrated in FIG. 4 comprises a hollow frame having an outer wall 78 and an inner wall 80. The inner wall 80 is provided with a plurality of apertures aligned with the plane surface of the flange 12 of the oven compartment 10, around the entire periphery of the flange 12. In each of the. apertures, there is disposed a shaft 82 which is secured to a closing member 84 disposed on the oppo-' site side of the rear plate 80. Within the door 18, a collar 86 is secured to each shaft 82, and a separate spring 88 forces each shaft 82 rearwardly by imparting a force between the front wall 78 of the door 18 and the collar 86. The closing plate 84 is formed of resilient material to permit it to substantially conform to the contour of the flange 12, if the contour differs from a plane surface. In this way, radiation leakage from the door when it is in closed position is substantially prevented.

The plane surface of the flange 12 is designed to be about wavelength of the microwave input frequency, to further attenuate any radiation which manages to leak between the flange 12 and the closing member 60.

A microswitch 90 (FIG. 2) is secured to the cabinet 14, and its actuator element cooperates with the rear Wall 80 of the door 18, when the door 18 is in closed position as illustrated in FIG. 2. The microswitch 90 therefore furnishes an indication of when the door is open and closed, and is connected in the magnetron circuit to prevent operation of the magnetron while the door is open, thereby to prevent radiation from escaping from the oven compartment when the door 18 is opened inadvertently during the cooking process.

The forward end of the wave guide 22 is provided with a movable wall 92 having a handle 94, which permits the length of the wave guide 22 to be adjusted in accordance with the desired operating point of the magnetron on its performance and characteristic curve. Once the wall 92 is adjusted initially, it may remain in that position without the necessity of any further adjustment. Its main function is merely to account for variations in size and geometry of the wave guide and oven compartment 10 which are the result of manufacturing tolerances. The tuning and impedance match of the system may be investigated by inserting a probe in the slot 91 in the wave guide 22, and examining the standing wave ratio.

The movable wall 92 is initially adjusted with an average load within the oven compartment 10 to give a standing wave ratio of 1 to 1 within the wave guide 22. Variations in load within the oven compartment 10 may cause the standing wave ratio to rise to approximately 2 to 1 during the cooking operation, although higher standing wave ratios are tolerable, depending upon the characteristics of the magnetron 44. A standing wave ratio of 1 to 1 indicates that all the transmitted power is being absorbed by the load, and is therefore the design objective.

The presence of the bafile plate 28 permits the oven compartment 10 with its load, in the form of food with in the compartment, to present a load impedance as seen by the magnetron of very nearly the characteristic output impedance of the magnetron which in a typical case is about 50 ohms. The baflle 28 also prevents over exposure of the top surface of the food to be cooked by preventing the incidence of radiation directly from the primary source represented by the aligned slots 24 and 26. The result is a more uniform field distribution throughout the entire thickness of the food to be cooked, which provides for substantially even cooking of the food.

In an exemplary embodiment of the present invention, the magnetron used was a 1 kw. MO-23 air cooled magnetron, produced by Deutche Mikrowellen of Freiburg, Germany, and the dimensions of some of the components were as follows: The frequency employed was 2450 megacycles, the oven compartment was 178 min. (1.5x) wide, 114 mm. (.93)\) high, and 213 mm. (1.7x) deep, the wave guide was 43 x 86 mm., the distance from the center of the stub to the closest end wall of the wave guide 22 was 22 mm., the center of the aperture 24 was 223 mm. from the center of the stub 48, and 57 mm. from the end wall 92 of the wave guide 22. The bafile plate 28 was supported 22.3 mm. (.18 below the top wall 27, and is therefore less than 91.7 mm. (.75 from the turntable 32, where A is the wavelength in free space of the microwave energy. As the edges of the baffle plate 28 are spaced from the side walls by about 31 mm. (25A), the baffle dimensions are about 116 mm. (1.0x) by 151 mm. (1.2x); and the volume of the compartment is 178 mm. x 114 mm. x 213 mm. or 4.3 cm. (2.4)@).

The foregoing will so fully and completely described the present invention as to enable those skilled in the art, by applying current knowledge, to readily adapt the same for use under varying conditions of service without departing from the essential features of novelty involved, which are intended to be defined and secured by the appending claims.

What is claimed is:

1. An electronic oven comprising a source of microwave energy, an oven compartment, and a wave guide having a closed end interconnecting said source and said compartment, said source being connected to said wave guide at a location spaced from said closed end thereof, a first aperture disposed in said wave guide between said location and said closed end of said Wave guide, means for directinga stream of air against said wave guide in the vicinity of said first aperture to cause a stream of air to flow through said aperture into said wave guide past said source, and a second aperture disposed in said wave guide on the side of said location remote from said first aperture for allowing said stream of air to flow out of said wave guide.

2. An electronic oven comprising a source of microwave energy, an oven compartment, means for introducing said energy in the form of radiation through an aperture into said compartment, dielectric support means for supporting material to be cooked within said compartment, and a bafile interposed between said material to be cooked and said aperture, said bafile being spaced from said support means by less than A of a wavelength of said microwave energy in free space.

3. An electronic oven comprising a source of microwave energy, an oven compartment, means for introducing said energy in the form of radiation through an aperture into said compartment, a dielectric platform adapted to support material to be cooked within said oven compartment, and a thin conductive bafile interposed between said platform and said aperture, said baffle being disposed closer to said aperture than to said platform, said bafile having an area which is of the same order of magnitude as the area of said platform, and the distance from said bafile to said platform being less than of a wavelength of said microwave energy in free space.

4. An electronic oven comprising a source of microwave energy, an oven compartment, means for introducing said energy in the form of radiation through an aperture into said compartment support means for supporting material to be cooked Within said oven compartment, and a baffle interposed between said material to be cooked and said aperture, said baffle being spaced from said aperture by about /s of a wavelength of microwave energy in free space, and from said support means by less than of a wavelength of said microwave energy in free space.

5. An electronic oven comprising a source of microwave energy, an oven compartment, means for introducing said energy in the form of radiation through an aperture into said compartment, support means for supporting material to be heated within said compartment, and a bafile interposed between said material to vbe cooked and said aperture, said oven compartment having a volume no greater than approximately 2.4 times a cube of the wavelength of said microwave energy in free space, whereby a food article occupying the major portion of said compartment is heated uniformly, and the energy distribution throughout said compartment during said heating is substantially uniform.

6. An electronic oven comprising a source of microwave energy, an oven compartment, means for introducing said energy in the form of radiation through an aperture into said-compartment, support means for supporting material to be heated within said oven compartment, and a baflle interposed between said material to be heated and said aperture, said bafiie being composed of a thin rectangular metallic sheet having dimensions of about 1.0x by 1.2x, where )t is the wavelength of said microwave energy in free space.

7. Apparatus according to claim 6 wherein said com- 7 8 partment is in the form of a rectangular parallelopiped, 2,909,635 10/1959 Haagensen 21910.55 and the edges of said baffie are spaced from the walls 3,127,495 3/1964 Polries et a1. 21910.55 of said compartment by about 025A. 3,182,166 5/1965 Bohm et a1 21910.55

References Cited by the Examiner 5 FOREIGN PATENTS 1,234,661 10/ 1960 France. UNITED STATES ljATENTs 1,249,130 11/1960 France. 1,831,013 11/1931 Kouyoumjlan 219--320 074 2 1952 Great Britain. 2,595,748 5/1952 Andrew-S 219-10.55 941 434 11/19 3 Great Britain 2,632,090 '3/1953 Revel-comb et a1. 21910.55

2,733,650 2/1956 Williams 219-105s RICHARD M. WOOD, Primary Examiner. 2,782,292 2/1957 Lon-g 219-4055 X 2,827,537 3/1958 Haa-gensen 219-1055 ANTHONY BARTIS Examm 2,831,952 4/1958 Warner 219-1055 L. H.BENDER,Assistant Examiner. 

1. AN ELECTRONIC OVEN COMPRISING A SOURCE OF MICROWAVE ENERGY, AN OVEN COMPARTMENT, AND A WAVE GUIDE HAVING A CLOSED END INTERCONNECTING SAID SOURCE AND SAID COMPARTMENT, SAID SOURCE BEING CONNECTED TO SAID WAVE GUIDE AT A LOCATION SPACED FROM SAID CLOSED END THEREOF, A FIRST APERTURE DISPOSED IN SAID WAVE GUIDE BETWEEN SAID LOCATION AND SAID CLOSED END OF SAID WAVE GUIDE, MEANS FOR DIRECTING A STREAM OF AIR AGAINST SAID WAVE GUIDE IN THE VICINITY OF SAID FIRST APERTURE TO CAUSE A STREAM OF AIR TO FLOW THROUGH SAID APERTURE INTO SAID WAVE GUIDE PAST SAID SOURCE, AND A SECOND APERTURE DISPOSED IN SAID WAVE GUIDE ON THE SIDE OF SAID LOCATION REMOTE FROM SAID FIRST APERTURE FOR ALLOWING SAID STREAM OF AIR TO FLOW OUT OF SAID WAVE GUIDE. 